THIS invention relates to an electronic device, in particular a transistor, and to methods of fabricating such a device.
This invention further relates to the field of printed electronics, which is the fabrication of electronic devices and circuitry using printing, coating and packaging techniques.
Transistors, being electronic devices which exhibit a transconductance or transresistance, are well known in the art. Commonly transistors are divided into two classes: junction transistors and field effect transistors (FETs). The principle of operation of the FET and a design for a junction FET were first disclosed by Lilienfeld in CA 272,437 on 22 Oct. 1925, and a variation, of the insulated gate FET (IG-FET) by Bardeen in U.S. Pat. No. 2,524,033, filed on 26 Feb. 1948. The bipolar junction transistor (BJT) was first disclosed by Shockley on the same date in U.S. Pat. No. 2,569,347. Working with Brattain, Bardeen and Shockley developed the point contact transistor (PCT), which was the first industrially applicable transistor, for which they received the 1956 Nobel Prize in Physics and which was disclosed in U.S. Pat. No. 2,524,035. In essence, all subsequent transistor developments have been improvements in the architecture, fabrication or materials of the FET and BJT.
Prior art transistors work along the same principle as a triode valve, or vacuum tube, of modulating the current between two terminals or electrodes, known as the emitter and collector (junction transistors), or source and drain (FET) by the presence of current at, or the application of a potential to, a third electrode known as the base (BJT) or gate (FET). Early applications of transistors were therefore signal amplifiers, in which the modulation of a small base current in a BJT is mapped in a linear relationship onto a larger emitter-collector current. In field effect transistors the main cause of the modulation of the source-drain current is the depletion or enhancement of the number of free charge carriers in the semiconductor material by the electric field resulting from the application of a potential to the gate. These classes of transistors are therefore highly suited to switching the source-drain current on or off, and have their main application in logic circuits, memory and display switching.
In printed electronics, most developments have been on insulated gate field effect transistors, using predominantly organic semiconducting materials. For printed inorganic semiconductors most working transistors have been insulated gate FETs, for example as described by Härting et al Appl. Phys Lett 94, 19193509 (2009), and metal semiconductor junction FETs, as disclosed in U.S. Pat. No. 8,026,565. The concept of a printed bipolar junction transistor is, however, a goal which still has to be realised, and has been disclosed by Schmid et al in U.S. Pat. No. 7,432,126, for example. This focus on field effect transistors has largely been driven by the expected applications, in which transistors are employed as logic gates and display drivers, in interactive packaging, supply chain security, radio frequency identification, and marketing. Progress in developing these applications has to some extent been hindered by the complexity of the printed transistor, which requires exact control of the properties, compatibility during processing, arrangement and thickness of multiple layers of different materials.